Abstract The cause of idiopathic Parkinson?s disease (PD) remains unknown, however, significant evidence suggests that interaction between genetic susceptibility and environmental factors is the predominant etiology of PD. Environmental toxicants that cause mitochondrial dysfunction, such as the organic pesticide rotenone, and the common herbicide paraquat, are associated with elevated PD risk (OR 2.5, 95% CI: 0.1.3-4.7; OR 2.5, 95% CI: 1.4-4.7; respectively). A heavily used industrial solvent, trichloroethylene (TCE), also causes mitochondrial toxicity, and is the most frequently reported organic contaminant found in US groundwater. TCE exposure is linked to the development of PD (OR 6.1, 95% CI: 1.2-3.3), and rodent models of TCE exposure display dopamine neuron degeneration from the nigrostriatal tract. Recent evidence from our lab indicates that rotenone (ROT), paraquat (PQ), and TCE interact with PD susceptibility genes, notably, causing the activation of LRRK2 in wildtype (WT) human embryonic kidney (HEK) cells, which could be blocked by a selective LRRK2 inhibitor (GNE-7915). As LRRK2 is the most commonly inherited mutation associated with familial PD, this evidence suggests that a gene-environment interaction exists between LRRK2 and mitochondrial toxicants. Functionally, LRRK2 activation leads to multiple downstream cellular pathologies, such as disruption of vesicular trafficking, deficits in autophagy, the phosphorylation of ?-synuclein, and neuroinflammation; all of which are mechanisms hypothesized to precede dopamine neuron degeneration in PD. The basis of this proposal is to investigate LRRK2 activation and pre-degenerative mechanisms in dopamine neurons caused by environmental mitochondrial toxicants. To achieve this, we will pursue the following specific aims: Aim 1 (K99) will build a foundation to identify if LRRK2 activity is induced in WT neurons by environmental mitochondrial toxicants, and if LRRK2 mutations exacerbate this pathology following mitochondrial dysfunction. Aim 2 (R00) will further characterize LRRK2 activation in an animal model of TCE exposure, and determine whether LRRK2 inhibition is protective against TCE. Aim 3 (R00) will characterize another pre-degenerative mechanism influenced by environment mitochondrial toxicants and LRRK2, mitochondrial antigen presentation (MitAP). MitAP involves the trafficking of mitochondrial proteins from the inner lumen to cell surface MHC molecules, causing the selective killing of dopamine neurons by immune cells. We have measured MitAP in dopamine neurons following a single exposure to ROT in rats, suggesting this is an early response to mitochondrial toxicity. We will identify whether ROT, PQ, and TCE induce MitAP in dopaminergic neurons. As LRRK2 activity affects vesicular trafficking, we propose that LRRK2 mutations in N27 cell lines influences MitAP in response to mitochondrial toxicant exposure, and treatment with a LRRK2 inhibitor may be protective against MitAP in vivo. These aims will provide innovative evidence for LRRK2 activation by environmental factors that contributes to dopaminergic neuron degeneration. Collectively, this proposal may lead to new therapeutic treatment avenues for idiopathic and inherited PD.